Abstract
Retarded plasmon eigenmodes in metallic nanoshells are theoretically analyzed, and both plasmon eigenfrequencies and plasmon decay rates are calculated. Spherelike and voidlike plasmon modes are considered and their behavior with geometrical parameters is analyzed. Special attention is given to the problem of radiative decay of different plasmon modes supported by such systems. It is concluded that by varying the shell-layer thickness, the voidlike plasmon decay time can be varied over more than two orders of magnitude throughout the femtosecond range. For shell layers thinner than the characteristic skin depth, the voidlike plasmon modes exhibit subfemtosecond radiative lifetimes and hence they become more radiative than spherelike ones. For shell-layer thickness exceeding the characteristic skin depth, the decay time of the voidlike plasmons becomes of the order of tens of femtoseconds, yielding ultrahigh local-field enhancements. We predict local-field enhancement factors that exceed 60 and 150 in gold and silver nanoshells, respectively. These results are supported by calculations of absorption cross sections of these shells to external light. The results are applied to qualitatively explain strong coupling of plasmons with light in nanocellular metallic films, recently observed in light reflection experiments.
Highlights
Electromagnetic eigenmodes of small metallic particles have attracted much attention in the past and are of great importance in current technology since they can be efficiently excited by light and their resonant frequencies can be tuned by varying the geometrical structure of metal nanoclasters and the dielectric properties of the host medium, or by choosing different metallic materials.1,2Plasma oscillations excited in metallic particles cause a large field enhancement of the local field inside and near the particle
To obtain the radiative decay rate of eigenmodes in the metallic shell, which is basically the parameter controlling the coupling between eigenoscillations and radiation field,30 we assume in our calculations ␥eϭ0 for a while
Only the electrical modes with qϭ1 will be considered. Such plasmon modes can be excited in a shell with radius that is smaller than the light wavelength
Summary
Electromagnetic eigenmodes of small metallic particles have attracted much attention in the past and are of great importance in current technology since they can be efficiently excited by light and their resonant frequencies can be tuned by varying the geometrical structure of metal nanoclasters and the dielectric properties of the host medium, or by choosing different metallic materials.. Plasmon excitations in gold nanoporous films have been experimentally observed and their remarkable photonic properties established It was presumed in Ref. 13 that plasmon modes excited in spherical nanocavities couple much more effectively to light than those in metallic spheres, which results in strong reflectivity resonances observed in experiment. That model gives the eigenfrequency values, which somehow can be fitted to the frequencies of resonances in the measured reflectivity spectra, it cannot describe the coupling between plasmon modes in the nanocavities and the radiation field. We estimate the local-field enhancement factor at plasmon resonances of a metallic nanoshell and show theoretically that for some nanoshell parameters the local-field enhancement factor in voidlike plasmon mode can reach ultrahigh values. We conclude that unique optical properties of metallic nanoshells pave the way towards various useful application of such particles in plasmon-resonance nanooptics
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